Two video cards primarily combine their GPU processing power to give you more power than the GPU on its own, however dual GPUs have a few setbacks:
1. VRAM is not added in multi-GPU setups, if you Xfire 2 1GB cards together, you will still only have 1GB VRAM because data is cloned on each card.
If the card in question is a dual GPU single PCB card (like 295x, your 250 is not), then effective VRAM is half of what it says on the box.
2. Games themselves have to support Xfire/SLI to make use of it. How much more fps you get from Xfire also depends on the game, some can get close to 100% scaling, some not so high. Some games do not support Xfire/SLI at all. Best to check each game's review to check Xfire compatibility.
3. Your motherboard has to have the proper SLI slots to XFire/SLI (SLI is pickier), an exact model of your model would be required to see if Xfire is an option for you, though I do not think H series is going to give you very good performance either way.
Also a personal opinion
250 is far too weak of a card to consider for Xfire, I would consider Xfire with a minimum of 280x, and only if you need the extra power. I would highly recommend a single card upgrade instead.